You should PM Autotrust on the forum, he's not as active, but back in the day he used to do some crazy things with gemmy's. He's in the SE burbs too and loves a yarn about gemmys.
The 2JZ in the AE86 is out of a 1993 supra I think (not sure) but its a late model engine anyways. They cost about $2500 with an auto gearbox, manual gearboxes are rare and expensive. The engine's actually bigger than an RB26, its a 3.0L... its just that the turbo's huge. If you want a toyota engine, then I'd look at a 1JZ turbo engine, which you should be able to pick up with a manual gearbox for around $2500.
The big issue with gemmy's and inline 6 engines is that the firewall needs to be modified. If you wanna do a nissan engine, then jam a VG30DETT engine in there... 3.0L twin turbo, heaps of torque. They're not as long as an RB so you should be able to get away with not modifying the firewall.